The median nerve is derived from the major cervical nerve roots (C5-8) and a minor first thoracic (T1) nerve root contribution. Within the axilla, various fascicles of these nerve roots join to form the lateral, medial, and posterior cords of the brachial plexus. Subsequently, a significant portion of the lateral and medial cords fuse to form the median nerve adjacent to the axillary artery. As the median nerve travels through the axilla and into the arm, it lies lateral to the brachial artery. Lower, near the coracobrachialis muscle insertion, the nerve moves medially over the brachial artery, descending toward the cubital fossa at the elbow. Occasionally, just above the elbow, the ligament of Struthers, a fibrous band extending from a small supracondylar spur to the medial epicondyle of the humerus, forms the roof of a tunnel for the median nerve and brachial artery to concomitantly pass through as they approach the elbow. Here the median nerve lies posterior to the bicipital aponeurosis (lacertus fibrosis), the intermediate cubital vein and superficial to the insertion of the brachialis muscle at the ulna tuberosity.
When performing venipuncture or arterial puncture, the close proximity of the median nerve to the intermediate basilic vein and brachial artery always must be considered. It is important to perform venipuncture immediately lateral to the bicipital tendon to avoid the brachial artery, which lies just medial to this prominent tendon. There are no significant median nerve motor or sensory branches within the arm.
The median nerve enters the forearm between the long and short heads of the biceps muscle. Initially, it innervates the pronator teres (PT) muscle (C6, 7). Subsequently, it innervates three other forearm muscles: flexor carpi radialis (FCR; C6, 7), palmaris longus (C7, 8, T1), and flexor digitorum superficialis (FDS; C7, 8). It also provides articular twigs to the elbow and proximal radioulnar joints.
Within the very upper forearm, the anterior interosseous nerve (AIN; the primary median nerve motor branch) is derived from the primary trunk of the median nerve. This is a primarily motor branch, coursing distally, superficial to the anterior interosseous ligament, accompanied by its anterior interosseous artery. The AIN innervates the lateral head of the flexor digitorum profundus (FDP; C7, 8) a muscle providing tendons that flex the most distal interphalangeal joints of the index and middle fingers, In addition, the AIN supplies the flexor pollicis longus (FPL; C7, 8, T1), which flexes the distal phalanx of the thumb, and the pronator quadratus (C7, 8, T1), which aids in wrist pronation. Thus the AIN, through its innervation of the FDP and the flexor pollicis longus (FPL), provides the essential means for allowing the most important very fine movements, leading to flexion of the most distal phalange of the index and middle fingers, and for allowing the thumb to make the all important pinch movement possible.
In the lower forearm, the main trunk of the median nerve lies deep to the FDS and superficial to the FDP. Eventually, the primary median nerve trunk becomes more superficial, lying between the tendons of the palmaris longus and the flexor carpi radialis, (FCR; C6, 7). Here the median palmar cutaneous branch originates, arising 3 to 4 cm above the flexor retinaculum and descending over this area to supply the skin of the median palm and the thenar eminence. This is the first and only median sensory branch that is defined before the median nerve enters the hand.
In the forearm, the median and ulnar nerves are occasionally interconnected by fibers passing between these nerves. The most common are the median-ulnar anastomosis (Martin-Gruber syndrome), wherein portions of the median nerve branch off within the forearm to join the ulnar nerve. Typically, when this ulnar nerve variant reaches the hand, these median fibers will subsequently innervate their appropriate median intrinsic muscles. This is important for electromyographers to recognize, especially when looking for ulnar nerve block at the elbow as discussed on the ulnar nerve plates.